High-speed and high-accuracy digital to analog converts (DACs) are important building blocks for many signal processing and telecommunication systems. A DAC is a device that converts a digital signal into an analog signal. Due to the ever increasing digital processing power and speed of modern chips, the need for DACs with higher sampling speeds is on the rise. For example, 3D high-definition televisions (HDTVs) use DACs with 200 Megasamples per second (MSPS) while telecommunication transmitters use DACs with over a few GSPS. In addition to the high sampling speed, many of these applications also require very high linearity and high Spurious-Free Dynamic Range (SFDR) in the output analog signal.
Generally DACs use multiple switches to steer current to one or multiple outputs. The switches are operated by digital signals generated by a chain of digital signal processing. As the switches are turned on or off by the digital signals, jitter in the digital signals to the switches may degrade linearity performance and add spurious noises at the analog output. With high-speed DACs, the situation is especially critical since jitter at the digital signals tends to cause a more pronounced effect as the frequency increases.
Another important factor in high-speed DACs is data dependent switching of switches. Data dependent switching can be caused partly due to the asymmetry in the beginning transition and ending transition of an “on” pulse (e.g., “1” pulse) and an “off” pulse (e.g., “0” pulse). Generally, the transition time for turning on the pulse and turning off the pulse are asymmetric. Due to such differences in transition time, data streams including combinations of “on” pulses and “off” pulses generate noise in the analog output of the DACs that is dependent on digital signals provided to the DACs. This results in formation of switching power from the power supply. This switching power, when interacting with package parasitic like bonding or routing inductances, can generate harmonics of the desired signal from power supply to outputs and can further degrade the SFDR and SNR of the DAC.
Further development in the area of DACs is therefore needed to address these issues.